Biology is confusing and I have a question I don't understand.?

[?wondering]

Cheetahs are not a healthy species. Several million years ago they were widespread in Africa and Asia, but their numbers fell drastically during their last ice age, and again when they were hunted to near extinction in the 19th century. Now, they suffer from low survivorship (a large number of animals dying), poor sperm quality, and greater susceptibility to disease. Normally, an animal will reject tissue transplanted from another animal, but cheetahs will not reject tissue grafted on to them from another cheetah.
What happened to the cheetah?
How did these bottleneck events affect their genetic variation?
How did the change in genetic variation affect the health of the species?
Where does genetic variation ultimately come from?
What mechanisms can maintain and increase genetic variation in natural populations?

 

[emucompboy]

LOL. How did Critterman get to be a top contributor in zoology?

> What happened to the cheetah?
The cheetah got bottlenecked down to there being a very few individuals alive at one time. All cheetahs today are descended from these very few, and thus the cheetah species as a whole is highly inbred. There is no "hybrid vigor" going on in this species because all the animals are very much related to each other -- as is shown by the nonrejection of tissue -- the immune systems are recognizing it as "self" even though it came from another animal.

> How did these bottleneck events affect their genetic variation?
With only a few individuals alive at the bottleneck events, genetic variation was pretty nearly eliminated. For most genes, there are no variant alleles at all. For a few, unfortunately, the survivors included individuals heterozygous for deleterious alleles. With inbreeding going on, the deleterious alleles are matching up with each other and being expressed, causing genetic diseases. See below.

> How did the change in genetic variation affect the health of the species?
It's pretty obvious that this species is unhealthy. Matching deleterious alleles are being expressed in a large proportion of individuals, thus "low survivorship (a large number of animals dying), poor sperm quality, and greater susceptibility to disease."

> Where does genetic variation ultimately come from?
Ultimately from mutations.
Mutations make new alleles.
Gene duplication events can keep a good copy of a gene around, while allowing a second copy to mutate and perhaps be repurposed.
Viruses can, rarely, carry useful genes from one organism to another through lysogeny. It happens.

> What mechanisms can maintain and increase genetic variation in natural populations?
Mutations make new alleles.
Immigration can bring new alleles to a semi-isolated population.
Sexual reproduction combines alleles in different ways.
Not being killed by people.

Obviously this works better if you have lots of populations rather than a few, and also if you're working with larger populations rather than smaller.